optical metrology system optimized with design goals

ABSTRACT

Provided is a method of designing an optical metrology system for measuring structures on a workpiece where the optical metrology system is configured to meet two or more design goals. The design of the optical metrology system is optimized by using collected design goal data in comparison to the set two or more design goals. In one embodiment, the optical metrology system is used for stand alone metrology systems. In another embodiment, the optical metrology system is integrated with a fabrication cluster in semiconductor manufacturing.

BACKGROUND

1. Field

The present application generally relates to the design of an opticalmetrology system to measure a structure formed on a work piece, and,more particularly, to a method of optimizing the design of an opticalmetrology system to meet two or more design goals.

2. Related Art

Optical metrology involves directing an incident beam at a structure ona workpiece, measuring the resulting diffraction signal, and analyzingthe measured diffraction signal to determine various characteristics ofthe structure. The workpiece can be a wafer, a substrate, photomask or amagnetic medium. In manufacturing of the workpieces, periodic gratingsare typically used for quality assurance. For example, one typical useof periodic gratings includes fabricating a periodic grating inproximity to the operating structure of a semiconductor chip. Theperiodic grating is then illuminated with an electromagnetic radiation.The electromagnetic radiation that deflects off of the periodic gratingare collected as a diffraction signal. The diffraction signal is thenanalyzed to determine whether the periodic grating and, by extension,whether the operating structure of the semiconductor chip has beenfabricated according to specifications.

In one conventional system, the diffraction signal collected fromilluminating the periodic grating (the measured diffraction signal) iscompared to a library of simulated diffraction signals. Each simulateddiffraction signal in the library is associated with a hypotheticalprofile. When a match is made between the measured diffraction signaland one of the simulated diffraction signals in the library, thehypothetical profile associated with the simulated diffraction signal ispresumed to represent the actual profile of the periodic grating. Thehypothetical profiles, which are used to generate the simulateddiffraction signals, are generated based on a profile model thatcharacterizes the structure to be examined. Thus, in order to accuratelydetermine the profile of the structure using optical metrology, aprofile model that accurately characterizes the structure should beused.

With increased requirement for throughput, decreasing size of the teststructures, smaller spot sizes, and lower cost of ownership, there isgreater need to optimize design of optical metrology systems to meetseveral design goals. Characteristics of the optical metrology systemincluding throughput, range of measurement capabilities, accuracy andrepeatability of diffraction signal measurements are essential tomeeting the increased requirement for smaller spot size and lower costof ownership of the optical metrology system.

SUMMARY

Provided is a method of designing an optical metrology system formeasuring structures on a workpiece where the optical metrology systemis configured to meet two or more design goals. The design of theoptical metrology system is optimized by using collected design goaldata in comparison to the set two or more design goals. In oneembodiment, the optical metrology system is used for standalonemetrology systems. In another embodiment, the optical metrology systemis integrated with a fabrication cluster in semiconductor manufacturing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an architectural diagram illustrating an exemplary embodimentwhere an optical metrology system can be utilized to determine theprofiles of structures formed on a semiconductor wafer.

FIG. 1B depicts an exemplary optical metrology system in accordance withembodiments of the invention.

FIG. 2 depicts an exemplary flowchart for designing a metrology systemfor extracting structure profile parameters and controlling afabrication process.

FIG. 3 depicts an exemplary flowchart for a system for extractingstructure profile parameters using measurements from the opticalmetrology system.

FIG. 4 depicts an exemplary flowchart for optimizing the design of anoptical metrology system based on two or more design goals.

FIG. 5 depicts a graph of a reflectance as a function of wavelength fora simulated diffraction signal and a measured diffraction signal.

FIG. 6A depicts a graph of an illumination beam intensity as a functionof distance from the center of a uniform beam whereas FIG. 6B depicts agraph of an illumination beam intensity from the center of anasymmetrical non-uniform beam.

FIG. 7 is an exemplary block diagram of a system to optimize the designof an optical metrology system using two or more design goals.

DETAILED DESCRIPTION

In order to facilitate the description of the present invention, asemiconductor wafer may be utilized to illustrate an application of theconcept. The systems and processes equally apply to other workpiecesthat have repeating structures. The workpiece may be a wafer, asubstrate, disk, or the like. Furthermore, in this application, the termstructure when it is not qualified refers to a patterned structure.

FIG. 1A is an architectural diagram illustrating an exemplary embodimentwhere optical metrology can be utilized to determine the profiles orshapes of structures fabricated on a semiconductor wafer. The opticalmetrology system 40 includes a metrology beam source 41 projecting ametrology illumination beam 43 at the target structure 59 of a wafer 47.The metrology beam 43 is projected at an incidence angle θ (label 45 inFIG. 1A) towards the target structure 59. The diffracted detection beam49 is measured by a metrology beam receiver 51. A measured diffractionsignal 57 is transmitted to a processor 53. The processor 53 comparesthe measured diffraction signal 57 against a simulator 60 of simulateddiffraction signals and associated hypothetical profiles representingvarying combinations of critical dimensions of the target structure andresolution. The simulator can be either a library that consists of amachine learning system, pre-generated data base and the like (e.g.,this is a library system), or on demand diffraction signal generatorthat solves the Maxwell equation for a giving profile (e.g., this is aregression system). In one exemplary embodiment, the diffraction signalgenerated by the simulator 60 instance best matching the measureddiffraction signal 57 is selected. The hypothetical profile andassociated critical dimensions of the selected simulator 60 instance areassumed to correspond to the actual cross-sectional shape and criticaldimensions of the features of the target structure 59. The opticalmetrology system 40 may utilize a reflectometer, an ellipsometer, orother optical metrology device to measure the diffraction beam orsignal. An optical metrology system is described in U.S. Pat. No.6,913,900, entitled “GENERATION OF A LIBRARY OF PERIODIC GRATINGDIFFRACTION SIGNAL”, issued on Sep. 13, 2005, which is incorporatedherein by reference in its entirety.

Simulated diffraction signals can be generated by applying Maxwell'sequations and using a numerical analysis technique to solve Maxwell'sequations. It should be noted that various numerical analysistechniques, including variations of rigorous coupled-wave analysis(RCWA), can be used. For a more detail description of RCWA, see U.S.Pat. No. 6,891,626, entitled “CACHING OF INTRA-LAYER CALCULATIONS FORRAPID RIGOROUS COUPLED-WAVE ANALYSES”, filed on Jan. 25, 2001, issuedMay 10, 2005, which is incorporated herein by reference in its entirety.

Simulated diffraction signals can also be generated using a machinelearning system (MLS). Prior to generating the simulated diffractionsignals, the MLS is trained using known input and output data. In oneexemplary embodiment, simulated diffraction signals can be generatedusing an MLS employing a machine learning algorithm, such asback-propagation, radial basis function, support vector, kernelregression, and the like. For a more detailed description of machinelearning systems and algorithms, see U.S. patent application Ser. No.10/608,300, entitled “OPTICAL METROLOGY OF STRUCTURES FORMED ONSEMICONDUCTOR WAFERS USING MACHINE LEARNING SYSTEMS”, filed on Jun. 27,2003, which is incorporated herein by reference in its entirety.

FIG. 1B shows an exemplary block diagram of an optical metrology systemin accordance with embodiments of the invention. In the illustratedembodiment, an optical metrology system 100 can comprise a lampsubsystem 105, and at least two optical outputs 106 from the lampsubsystem can be transmitted to an illuminator subsystem 110. At leasttwo optical outputs 111 from the illuminator subsystem 110 can betransmitted to a selector subsystem 115. The selector subsystem 115 cansend at least two signals 116 to a beam generator subsystem 120. Inaddition, a reference subsystem 125 can be used to provide at least tworeference outputs 126 to the beam generator subsystem 120. The wafer 101is positioned using an X-Y-Z-theta stage 102 where the wafer 101 isadjacent to a wafer alignment sensor 104, supported by a platform base103.

The optical metrology system 100 can comprise a first selectablereflection subsystem 130 that can be used to direct at least two outputs121 from the beam generator subsystem 120 on a first path 131 whenoperating in a first mode “LOW AOI” (AOI, Angle of Incidence) or on asecond path 132 when operating in a second mode “HIGH AOI”. When thefirst selectable reflection subsystem 130 is operating in the first mode“LOW AOI”, at least two of the outputs 121 from the beam generatorsubsystem 120 can be directed to a first reflection subsystem 140 asoutputs 131, and at least two outputs 141 from the first reflectionsubsystem can be directed to a high angle focusing subsystem 145. Whenthe first selectable reflection subsystem 130 is operating in the secondmode “HIGH AOI”, at least two of the outputs 121 from the beam generatorsubsystem 120 can be directed to a low angle focusing subsystem 135 asoutputs 132. Alternatively, other modes in addition to “LOW AOI” and“HIGH AOI” may be used and other configurations may be used.

When the metrology system 100 is operating in the first mode “LOW AOI”,at least two of the outputs 146 from the high angle focusing subsystem145 can be directed to the wafer 101. For example, a high angle ofincidence can be used. When the metrology system 100 is operating in thesecond mode “HIGH AOI”, at least two of the outputs 136 from the lowangle focusing subsystem 135 can be directed to the wafer 101. Forexample, a low angle of incidence can be used. Alternatively, othermodes may be used and other configurations may be used. The opticalmetrology system 100 can comprise a high angle collection subsystem 155,a low angle collection subsystem 165, a second reflection subsystem 150,and a second selectable reflection subsystem 160.

When the metrology system 100 is operating in the first mode “LOW AOI”,at least two of the outputs 156 from the wafer 101 can be directed tothe high angle collection subsystem 155. For example, a high angle ofincidence can be used. In addition, the high angle collection subsystem155 can process the outputs 156 obtained from the wafer 101 and highangle collection subsystem 155 can provide outputs 151 to the secondreflection subsystem 150, and the second reflection subsystem 150 canprovide outputs 152 to the second selectable reflection subsystem 160.When the second selectable reflection subsystem 160 is operating in thefirst mode “LOW AOI” the outputs 152 from the second reflectionsubsystem 150 can be directed to the analyzer subsystem 170. Forexample, at least two blocking elements can be moved allowing theoutputs 152 from the second reflection subsystem 150 to pass through thesecond selectable reflection subsystem 160 with a minimum amount ofloss.

When the metrology system 100 is operating in the second mode “HIGHAOI”, at least two of the outputs 166 from the wafer 101 can be directedto the low angle collection subsystem 165. For example, a low angle ofincidence can be used. In addition, the low angle collection subsystem165 can process the outputs 166 obtained from the wafer 101 and lowangle collection subsystem 165 can provide outputs 161 to the secondselectable reflection subsystem 160. When the second selectablereflection subsystem 160 is operating in the second mode “HIGH AOI” theoutputs 162 from the second selectable reflection subsystem 160 can bedirected to the analyzer subsystem 170.

When the metrology system 100 is operating in the first mode “LOW AOI”,high incident angle data from the wafer 101 can be analyzed using theanalyzer subsystem 170, and when the metrology system 100 is operatingin the second mode “HIGH AOI”, low incident angle data from the wafer101 can be analyzed using the analyzer subsystem 170.

Metrology system 100 can include at least two measurement subsystems175. At least two of the measurement subsystems 175 can include at leasttwo detectors such as spectrometers. For example, the spectrometers canoperate from the Deep-Ultra-Violet to the visible regions of thespectrum.

The metrology system 100 can include at least two camera subsystems 180,at least two illumination and imaging subsystems 182 coupled to at leasttwo of the camera subsystems 180. In addition, the metrology system 100can also include at least two illuminator subsystems 184 that can becoupled to at least two of the imaging subsystems 182.

In some embodiments, the metrology system 100 can include at least twoauto-focusing subsystems 190. Alternatively, other focusing techniquesmay be used.

At least two of the controllers (not shown) in at least two of thesubsystems (105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160,165, 170, 175, 180, 182, 190, and 195) can be used when performingmeasurements of the structures. A controller can receive real-signaldata to update subsystem, processing element, process, recipe, profile,image, pattern, and/or model data. At least two of the subsystems (105,110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175,180, 182, and 190) can exchange data using at least two SemiconductorEquipment Communications Standard (SECS) messages, can read and/orremove information, can feed forward, and/or can feedback theinformation, and/or can send information as a SECS message.

Those skilled in the art will recognize that at least two of thesubsystems (105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160,165, 170, 175, 180, 182, 190, and 195) can include computers and memorycomponents (not shown) as required. For example, the memory components(not shown) can be used for storing information and instructions to beexecuted by computers (not shown) and may be used for storing temporaryvariables or other intermediate information during the execution ofinstructions by the various computers/processors in the metrology system100. At least two of the subsystems (105, 110, 115, 120, 125, 130, 135,140, 145, 150, 155, 160, 165, 170, 175, 180, 185, and 190) can includethe means for reading data and/or instructions from a computer readablemedium and can comprise the means for writing data and/or instructionsto a computer readable medium. The metrology system 100 can perform aportion of or all of the processing steps of the invention in responseto the computers/processors in the processing system executing at leasttwo sequences of at least two instructions contained in a memory and/orreceived in a message. Such instructions may be received from anothercomputer, a computer readable medium, or a network connection. Inaddition, at least two of the subsystems (105, 110, 115, 120, 125, 130,135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 182, and 190) cancomprise control applications, Graphical User Interface (GUI)components, and/or database components.

It should be noted that the beam when the metrology system 100 isoperating in the first mode “LOW AOI” with a high incident angle datafrom the wafer 101 all the way to the measurement subsystems 175,(output 166, 161, 162, and 171) and when the metrology system 100 isoperating in the second mode “HIGH AOI” with a low incident angle datafrom the wafer 101 all the way to the measurement subsystems 175,(output 156, 151, 152, 162, and 171) is referred to as diffractionsignal(s).

FIG. 2 depicts an exemplary flowchart for designing an optical metrologysystem for extracting structure profile parameters and controlling afabrication process for semiconductors (or integrated circuits, orelectronic devices). In this exemplary embodiment, the optical metrologysystem is integrated in a fabrication cluster. It is understood that theoptical metrology system may be in a standalone metrology system with orwithout automated equipment delivering and retrieving work pieces fromthe optical metrology system. In step 204, an optical metrology systemcoupled to a fabrication cluster is designed to meet two or more designgoals. The fabrication cluster may be a lithography, etch, cleaning,chemical-mechanical polishing fabrication cluster, deposition cluster,or the like. The optical metrology system includes an optical metrologyfool such as a spectroscopic reflectometer, spectroscopic ellipsometer,hybrid optical device, and the like. The detail steps for designing theoptical metrology system are included in the description associated withthe flowchart, in FIG. 4.

Referring to FIG. 2, the two or more design goals may include: accuracyof the measured diffraction signals assessed by comparing the profileparameter determined from the measured diffraction signal to the profileparameter determined using a reference tool such as scanning electronmicroscope (SEM); repeatability of the measured diffraction signals,either static or dynamic repeatability or both, typically measured asstatistical variation from mean; range of spot sizes of illuminationbeam that can be measured by the optical metrology system; range ofsizes of the measurement spot; throughput in the number of workpiecesmeasured per unit time; and types and range of applications measured.Types and range of applications measured may include line and space orother one dimensional repeating structures, complex transistorstructures, two dimensional repeating structures such as vias, contactholes, posts, and trenches, structures with surface, edge, or shaperoughness, irregularly shaped structures such as structures havingpeanut-shaped islands, conical structures, structures with convex orconcave surfaces, structures in multiple layers such as overlay andchemical-mechanical polishing (CMP) applications, and other complexstructures such as double patterning structures, multiple pitchstructures or iso-dense structures. In alternate embodiments, designgoals may also include tool-to-tool matching ranges to a similar tool orto a reference tool or to a fleet of tools, reliability of the opticalmetrology system expressed as up time or mean time between failures,time needed to develop libraries for extracting profile parameters ortime needed to train machine learning systems for extracting profileparameters, or cost of ownership measured in dollars needed for service,repair cost, and maintenance costs, and the like. In one embodiment, theoptical metrology system may either be integrated in a process tool suchas an etcher or be part of a stand alone metrology module.

Still referring to FIG. 2, in step 208, a structure is measured with thedesigned optical metrology system generating a diffraction signal. Asmentioned above, the workpiece may be a wafer, a substrate, disk,photomask or the like. In step 212, at least one profile parameter ofthe structure is extracted from the measured diffraction signal usingone or more systems, such as the regression system, the library systemor the machine learning system described above. In step 216, at leastone extracted profile parameter of the structure is transmitted to thefabrication cluster. Extracted profile parameters may include criticaldimensions such as bottom width, top width or sidewall angle of thestructure. In other embodiments, extracted profile parameters mayinclude any profile parameter of the structure. In step 220, at leastone process parameter or equipment setting of the fabrication cluster isadjusted based on the at least one transmitted profile parameters.

FIG. 3 depicts an exemplary flowchart for a system for extractingprofile parameters using measurements from the optical metrology system.In step 254, an optical metrology model is developed using the profilemodel of the structure and the designed optical metrology system. Asmentioned above, the profile of the structure may be a simple line andspace grating or a more complex group of repeating structures such asposts, contact holes, vias, or combinations of different shapesstructures in a repeating pattern of unit cells. For a detaileddescription of modeling two-dimensional repeating structures, refer toU.S. patent application Ser. No. 11/061,303, entitled “OPTICAL METROLOGYOPTIMIZATION FOR REPETITIVE STRUCTURES”, by Vuong, et al., filed on Apr.27, 2004, and is incorporated in its entirety herein by reference. Theoptical metrology model includes characterization of the illuminationbeam that is used to illuminate the structure and characterization ofthe detection beam diffracted from the structure.

In step 258, a regression algorithm is developed to extract the profileparameters of the structure profile using measured diffraction signals.Typically, the regression algorithm compares a series of simulateddiffraction signals generated from a set of profile parameters where thesimulated diffraction signal is matched to the measured diffractionsignal until the matching criteria are met. For a more detaileddescription of a regression-based process, see U.S. Pat. No. 6,785,638,entitled “SYSTEM AND SYSTEM: FOR DYNAMIC LEARNING THROUGH AREGRESSION-BASED LIBRARY GENERATION PROCESS”, filed on Aug. 6, 2001,which is incorporated herein by reference in its entirety.

In step 262, a library of pairs of simulated diffraction signals andprofile parameters of the structure are developed. For a more detaileddescription of an exemplary library-based process, see U.S. Pat. No.6,943,900, entitled “GENERATION OF A LIBRARY OF PERIODIC GRATINGDIFFRACTION SIGNALS”, issued on Sep. 13, 2005, which is incorporatedherein by reference in its entirety. In step 266, an MLS trained usingpairs of simulated diffraction signals and profile parameters aredeveloped. The trained MLS is configured to generate a set of profileparameters as output based on an input measured diffraction signal. Fora more detailed description of a generating and using a trained MLS, seeU.S. Pat. No. 7,280,229, entitled “EXAMINING A STRUCTURE FORMED ON ASEMICONDUCTOR WAFER USING MACHINE LEARNING SYSTEMS”, filed on Dec. 3,2004, which is incorporated herein by reference in its entirety. In step270, at least one profile parameter of the structure profile isdetermined using the regression algorithm, the library, and/or thetrained MLS. It should be noted that the steps described above, (254,258, 262, 264, 268, and 270), apply to an optical metrology system in afabrication cluster or to a standalone optical metrology system.

FIG. 4 depicts an exemplary flowchart for optimizing the design of anoptical metrology system based on achieving two or more design goals. Instep 300, the range of capabilities of the optical metrology system isdetermined. The range of capabilities of the optical metrology systemmay include the types of workpieces or in the case of semiconductorworkpieces, wafer applications that can be measured which in turndetermines the number and type of measurement beams and optical paths,the range of illumination angles of incidence, number of measurementsites per wafer, the number of measurements per site, and the like. Forexample, if an optical metrology system is designed to measureone-dimensional repeating structures comprising lines and spaces, twomeasurement beams may be specified and the illumination beam ofincidence would more likely use a fixed angle of incidence. If anoptical metrology system is designed to measure both one-dimensionalrepeating structures and complex two-dimensional repeating structures,two or more measurement beams with a range of illumination beam anglesof incidence may be specified. Furthermore, the range of capabilitiesdetermined for the optical metrology system may require different typesof illumination devices, types of beam focusing optics, types ofpolarization of the beams, types of collection beam equipment,detectors, and processors.

In step 304, an initial design of the optical metrology system isdeveloped based on the range of capabilities determined in the step 300.The initial design includes components of the optical metrology systemcomprising light sources, a homogenizer to produce a uniform light spot,focusing optics for the illumination beams and coating specification forthe focusing optics, polarizers for the illumination beams and defectionbeams, a motion control system for moving the workpiece during patternrecognition and diffraction signal measurement, collecting optics forthe detection beams, at least two detectors for measuring thediffraction signals and efficiency of the detector gratings, use of anitrogen-purged system, a first processor for converting the measureddiffraction output to diffraction data, data storage for storing profileparameter extraction algorithms, libraries, or trained machine learningsystems, and a second processor for extracting at least one parameter ofthe structure from the diffraction signal. Furthermore, the measureddiffraction signal may be processed to increase the signal to noiseratio by using the first processor and algorithms for minimizingsystematic noise from the signal.

Referring to FIG. 4, in step 308, two or more design goals for opticalmetrology system are set. As mentioned above, the design goals caninclude accuracy of the measured diffraction signals compared to areference tool such as a SEM, an AFM (atomic force microscope) and thelike. Accuracy may also be measured in comparison to a previouslycalibrated scatterometry tool. Another design goal is repeatability ofthe measured diffraction signals, either static or dynamic repeatabilityor both, typically measured as statistical variation from the mean.Another design goal is the range of spot sizes of illumination beam thatcan be measured by the optical metrology system as well as the range ofsizes of the measurement spot. The optical metrology system may bedesigned to be able to measure a spot size of 32 by 32 micron orsmaller. The throughput in the number of workpieces measured per unittime is another design goal. The throughput can be number of workpiecesmeasured per unit time, such as an hour. As mentioned above, the opticalmetrology system may either be integrated in a process tool such as anetcher or be part of a stand alone metrology module. Another design goalis the type of applications that can be measured by the opticalmetrology system. As mentioned above, types of applications measured mayinclude one dimensional repeating structures, complex transistorsstructures, two dimensional repeating structures, structures withsurface, edge, or shape roughness, irregular shapes such as peanutshaped islands, convex or concave surfaces, multiple layers structuressuch as overlay and structures prior to chemical-mechanical polishing,complex structures such double patterning structures, multiple pitchstructures or iso-dense structures. Further design goals may alsoinclude tool-to-tool matching ranges to a similar tool or to a referencetool or to a fleet of tools, reliability of the optical metrology systemexpressed as up time or mean time between failures, time needed todevelop libraries for extracting profile parameters or time needed totrain machine learning systems for extracting profile parameters, orcost of ownership measured in dollars needed for service, repair cost,and maintenance costs, and the like.

In step 312, a metrology model for the optical metrology system isdeveloped. The metrology model includes components of the opticalmetrology system that have a functional association with set two or moredesign goals. Assume the two or more design goals include a throughputrate of at least 200 wafers per hour and an accuracy of structuremeasurement of 3 nanometers (nm) or less compared to cross-section SEM.For the throughput rate, the metrology model can include componentsassociated with the time budget of measuring a wafer for a givenapplication, the number of measurement sites, the number of measurementsper site, speed of moving the wafer or the measurement optics to thesite, alignment of the site, focusing of the beam, collection of thediffraction signal, processing of the diffraction signal, extraction ofprofile parameters such critical dimension (CD), side-wall angle, orwidth of the structure. For a detailed discussion of an opticalmetrology model designed to optimize an operating time budget, refer toU.S. patent application Ser. No. 12/050,053, entitled “METHOD OFDESIGNING AN OPTICAL METROLOGY SYSTEM OPTIMIZED FOR OPERATING TIMEBUDGET”, by Tian, et al., filed on Mar. 15, 2008 and is incorporated inits entirety herein by reference.

Referring to FIG. 4, step 312, the metrology model for the example willalso include components of the optical metrology system that have afunctional association with the accuracy of the measurement as expressedin the difference between the profile parameter extracted using themeasured diffraction signal and one measured using a cross-section SEM,where the difference is 3 nm or less. One operating characteristic ofthe optical metrology system that is correlated with accuracy of themeasurement includes intensity of the illumination and detection beamsfrom the one or more light sources up to and including the detectionsubsystem. Another operating characteristic of the optical metrologysystem that is correlated to accuracy of the measurement includes signalto noise ratio (SNR) of the illumination signal, the detection signal orboth. The SNR is affected by the type of optical components used and theenvironment in which the optical components operate in, such as the useof an optical environment purged with nitrogen gas, use of a homogenizerto provide highly uniform illumination, use of temperature controlsubsystems to keep the temperature of the light sources within a narrowtemperature range, use of a motion control system that minimizesvibrations of the chuck and workpiece, and the like. In otherembodiments, the SNR may also be enhanced using software algorithms toreduce the noise in the measured diffraction signal. Use of algorithmsto reduce the noise in the measured diffraction signal are described inU.S. patent application Ser. No. 12/018,028, entitled “NOISE-REDUCTIONMETROLOGY MODELS”, by Li, et al., filed on Jan. 22, 2008 and U.S. patentapplication Ser. No. 11/371,752, entitled “WEIGHTING FUNCTION TO ENHANCEMEASURED DIFFRACTION SIGNALS IN OPTICAL METROLOGY”, by Vuong, et al.,filed on Mar. 8, 2006, and are incorporated in their entirety herein byreference.

In step 316 of FIG. 4, a prototype of the optical metrology model isdeveloped. The prototype may include two or more of the metrologycomponents coupled so as to simulate the actual connections and settingsin the actual devices in production. For example, the prototype maycomprise the light sources and optical components up to and includingthe focusing optics in the illumination optical path. In anotherembodiment, the prototype may comprise the light sources, focusingoptical components, polarizers, and other optics in the illuminationoptical path, collection optical components, polarizers, and otheroptics in the detection optical path. In still another embodiment, theprototype may comprise all optical components from the light sources allthe way to the detectors. In still another embodiment, the prototype mayinclude a fully assembled optical metrology system.

In step 320, the data associated with the two or more design goals arecollected. For example, assume the two or more design goals include athroughput rate of at least 200 wafers per hour and an accuracy of 3nanometers (nm) or less compared to cross-section SEM measurements. Asmentioned above, the data collected to determine the throughput ratecomprises the total time budget needed to complete the measurement ofall the sites for the wafer. Time budget applies to metrology steps thatcannot be overlapped with other steps in the metrology cycle and thetotal time budget for each wafer is converted to the equivalentthroughput rate, such as wafers per hour. For a detailed discussion ofan optical metrology model designed to optimize an operating timebudget, refer to U.S. patent application Ser. No. 12/050,053, entitled“METHOD OF DESIGNING AN OPTICAL METROLOGY SYSTEM OPTIMIZED FOR OPERATINGTIME BUDGET”, by Tian, et al., filed on Mar. 18, 2008, and isincorporated in its entirety herein by reference. Also, as mentionedabove, accuracy of measurement in comparison to a reference metrologytool is a function of the intensity of the light sources, illuminationbeams, and detection beams. Accuracy is also affected by the signal tonoise ratio of the metrology beams. For a detailed discussion of anoptical metrology system optimized using operating criteria such assignal intensity and signal to noise ratio, refer to U.S. patentapplication Ser. No. 12/057,316, “DESIGNING AN OPTICAL METROLOGY SYSTEMOPTIMIZED WITH SIGNAL CRITERIA”, by Tian, et al., filed on Mar. 27, 2008and is incorporated in its entirety herein by reference.

In step 324, collected design goal data are compared to the set two ormore design goals. If the set two or more design goals are not met, instep 328, the design of the optical metrology system is modified andsteps 312, 316, 320, 324, and 328 are iterated until the two or moredesign goals are met. In step 328, modification of the design of theoptical metrology system depends on the set two or more design goals. Asin the example above, the two or more design goals include measurementof a spot size of 32 by 32 microns or less and a repeatability ofmeasurements with a 3-sigma variance of 2 nanometers or less. For thespot size design goal, modification of the design can include changingthe numerical aperture of a set of optical components by substitutingdifferent optics and/or using a different optics vendor. Othermodifications of the design can include changing the aperture to allowmore or less light or changing the shape of the aperture. Otherembodiments include design modifications that includes use of apodizers,changing the light source to use different kinds of bulbs, changing thelight source or using a different light source from a different vendor,using different size of lenses, changing the aperture shape, changingthe angle of incidence of the illumination beam closer to normal or viceversa, using a combination of large aperture optics and a small apertureslit, and the like. For the repeatability design goal, modification ofthe design can include the same design changes to increase the SNRmentioned above. In addition, design modifications can include keepingthe light source in a narrow range of temperature, increasing theaccuracy of the auto focus subsystem, increasing the precision of thepolarizer rotating mechanism, reducing the electronic noise in thedetection system, increasing the loading and positioning accuracy of theworkpiece loading mechanism, increasing the wafer alignment accuracy,and the like.

Other design modifications can include selecting two or more lightsources utilizing different ranges of wavelengths instead of utilizingone light source, illuminating the structure at substantially the samespot with the two or more beams from the two or more light sources atthe same time, measuring the two or more diffraction signals off thestructure and using one or more detectors for each of the two or morediffraction signals; selecting an off-axis reflectometer wherein theangle of incidence of the illumination beam is substantially around 28degrees instead of a normal or near normal angle of incidence; selectingan off-axis reflectometer wherein the angle of incidence of theillumination beam is substantially around 65 degrees instead of a nearnormal reflectometer or instead of 28 degrees; or reducing the number ofoptical components needed to implement the design.

Still referring to step 328, modification of the design of the of theoptical metrology system can also include using a selectable angle ofincidence for the illumination beam to optimize accuracy of thediffraction measurement instead of a fixed angle of incidence of theillumination beams, higher efficiency grating and higher efficiencysignal detector, configurable numerical aperture for the focusingoptics, light source, and the like. In other embodiments, modificationof the design of the of the optical metrology system can includeselecting a first polarizer in the illumination path and a secondpolarizer (or analyzer) in the detection path, wherein the first andsecond polarizers are configured to increase the signal to noise ratioof the illumination and detection beams respectively instead of regularpolarizers or substituting the first polarizer and the second polarizerwith polarizers from another vendor, replacing mirrors and focusingoptics with different quality coatings, replacement of diffractive opticwith reflective optics, and the like.

In one embodiment mentioned above, one of the two or more design goalsmay be used to qualify the accuracy of the measurement by comparing to areference measurement such as an AFM or a cross-section SEM measurement.Modification of the design can include minimizing the effect of systemnoise and artifacts in the measurement process. Referring to FIG. 5, areflectance graph 500 is depicted as a function of wavelength. In thereflectance graph 500, a simulated reflectance signal 504 and a measureddiffraction signal 508 are provided. The simulated reflectance signal504 is based on a model of the optical metrology system, the modelincluding a profile model of the structure on the workpiece and themodel of the optical metrology tool. Simulation of the reflectancesignal is performed, for example, using a numerical analysis solution ofthe Maxwell equations on electromagnetic diffraction such as RCWA. Themeasured diffraction signal 508 tracks the simulated reflectance signal504 except for artifacts 512, 514, and 518. The artifacts 512, 514, and518 may be due to a variety of causes such as polarizer leakage due toadditional or residual polarization or polarization caused byimperfections or contamination of the structure surface, scattering ofthe detection beam due to surface roughness of the structure,misalignment of an optical component, and the like. Modifications of thedesign of the optical metrology system may include replacement of thepolarizer, calibration of optical components to minimize the effect ofdeviations in the specified tolerance and/or imperfections of thecomponents during manufacture, or changes to the structure model toincorporate the surface roughness of the structure.

Still referring to FIG. 5, another cause for artifacts of the measureddiffraction signal can be due to lack of symmetry or uniformity in theintensity of the illumination beam. Referring to FIG. 6A, the graphicalillustration 600 of the illumination beam intensity as a function of thedistance from a center 604 of the illumination beam. An intensity graph608 is substantially a Gaussian distribution where the intensity of theillumination beam diminishes from the center 604 of the beam. Referringto FIG. 6B, a graphical illustration 650 of the illumination beamintensify as a function of the distance from a center 654 of theillumination beam shows irregularity of an intensity graph 658 where theintensity of the illumination beam dips in around the center 654 of theillumination beam. If the illumination beam included in the opticalmetrology tool model assumed a symmetrical and uniform intensity beam,the design change to increase accuracy of measurements may includechanging the model to incorporate the unevenness of the illuminationintensity, doing additional calibration of the light sources and opticalcomponents to minimize the residual effects of misalignment,imperfections during manufacture, and/or contamination during handling,or replacing the light source or using a different light source model oranother light source vendor, or using a beam homogenizer.

FIG. 7 is an exemplary block diagram of a system 700 to optimize thedesign of the optical metrology system to meet two or more design goals.The system 700 comprising an optical metrology system model 704, adesign goal data collector 708, a prototype 712, and a design goalanalyzer 716 are coupled to collect and optimize the optical metrologysystem of a particular design based on the set of two or more designgoals. The optical metrology system model 704 is based on an initialdesign of the optical metrology system. The initial design is driven bythe range of capabilities of the optical metrology system. As mentionedabove, the range of capabilities includes types of applications measuredand may include one dimensional repeating structures, complextransistors structures, two dimensional repeating structures, andcomplex structures such double patterning structures, multiple pitchstructures or iso-dense structures. Included in the optical system model704 are components of the optical metrology system that are functionallyassociated with the two or more design goals. The prototype 712comprises optical metrology system components that are configured tosimulate the performance of the actual optical metrology system. Theprototype 712 for an optical metrology system, two or more of the actualmetrology components are utilized to test out the optical path andconnections between mechanical and electronic components. For example,the prototype may include a motion control subsystem (not shown)programmed to position the wafer to the selected measurement sites,focusing subsystems in the illumination and detection optical paths, anda pattern recognition subsystem (not shown) to determine the orientationof the wafer, where the pattern recognition subsystem is coupled to themotion control subsystem.

Referring to FIG. 7, design goal data 721, for example, signal intensityor SNR at different points in the optical path measured in the prototype712 are transmitted to the design goal data collector 708. In addition,design goal data from the vendors or historical design goal data 731 forsimilar optical components are input into the design goal data collector708, and collections of design goal data 723 are further sent to theoptical metrology system model 704. The collections of design goal data723 are processed by the optical metrology system model 704 to generatedesign goal data for each design goal 725 and are transmitted to thedesign goal analyzer 716. The design goal analyzer 716 compares the twoor more design goals collected and assembled from all the sources tocorresponding input two or more design goals 729. Based the results ofthe comparison in the design goal analyzer 716, modifications to theoptical metrology system design 727 are determined and transmitted toand implemented in the prototype 712.

As mentioned above, modification of the design of the optical metrologysystem depends on the set two or more design goals. As in the exampleabove, assume the two or more design goals include measurement of a spotsize of 32 by 32 microns or less and a repeatability of measurementswith a 3-sigma variance of 2 nanometers or less. For the spot sizedesign goal, modification of the design can include changing thenumerical aperture of a set of optical components by substitutingdifferent optics, and/or using a different optics vendor. Othermodifications of the design can include changing the slit to allow moreor less light or changing the shape of the slit. Other embodimentsinclude design modifications including use of apodizers, changing thelight source to use different kinds of bulbs, changing the light sourceor using a different light source from a different vendor, usingdifferent size offenses, changing the angle of incidence of theillumination beam closer to normal or vice versa, using a combination oflarge aperture optics and a small aperture slit, and the like. For therepeatability design goal, modification of the design can include thesame design changes to increase the SNR mentioned above. In addition,design modifications can include keeping the light source in a narrowrange of temperature, increasing the accuracy of the auto focussubsystem, increasing the precision of the polarizer rotating mechanism,reducing the electronic noise in the detection system, increasing theloading and positioning accuracy of the workpiece loading mechanism,increasing the wafer alignment accuracy, and the like. Othermodification of the design of the optical metrology system can alsoinclude using a selectable angle of incidence for the illumination beamto optimize accuracy of the diffraction measurement instead of a fixedangle of incidence of the illumination beams; higher efficiency gratingand higher efficiency signal detector, configurable numerical aperturefor the focusing optics, light source, and the like.

In the other embodiment mentioned above, one of the two or more designgoals may be accuracy of the measurement compared to a referencemeasurement such as an AFM or a cross-section SEM. Modification of thedesign can include changing the model of the optical metrology toolincluded in the optical metrology system model 704. Steps to increaseaccuracy can include minimizing the effect of system noise and artifactsin the measurement process discussed in connection with FIG. 5. Also, asmentioned above, modifications of the design of the optical metrologysystem may include replacement of the polarizer, calibration of opticalcomponents to minimize the effect of deviations in the specifiedtolerance and/or imperfections of the components during manufacture.

Referring to FIG. 7, the changes to the design of the prototype 712 arealso incorporated in the design goal data collector 708 and into theoptical metrology system model 704. The set of design goal data 721 fromthe prototype 712 and from vendor data 731 are input into the designgoal data collector 708 and transmitted to the optical metrology systemmodel 704, generating new design goal data to compare to the set two ormore design goals. Changes to the design of the prototype 712, updatesto the design goal data collector 708 and to the optical metrologysystem model 704, and processing in the design goal analyzer 716 areiterated until the two or more design goals are met.

Although exemplary embodiments have been described, variousmodifications can be made without departing from the spirit and/or scopeof the present invention. For example, although throughput, spot size,accuracy, and repeatability of measurements were primarily used todescribe the embodiments of the invention, other design goals may alsobe used. Furthermore, the elements required for the design of theoptical metrology system are substantially the same whether the opticalmetrology system is integrated in a fabrication cluster or used in astandalone metrology setup. Therefore, the present invention should notbe construed as being limited to the specific forms shown in thedrawings and described above.

1. A method of optimizing the design of an optical metrology system, theoptical metrology system measuring structures on a workpiece, theoptical metrology system configured to meet two or more design goals,the method comprising: developing an initial design for the opticalmetrology system based on a determined range of capabilities of theoptical metrology system; setting two or more design goals for theoptical metrology system to a corresponding value or range of values;developing a model for the optical metrology system using the initialdesign and the set two or more design goals for the optical metrologysystem; developing a prototype of the optical metrology system based onthe initial design of the optical metrology system; collecting measureddesign goal data using the optical metrology system prototype and themodel of the optical metrology system; and if two or more measureddesign goals derived from the collected design goal data do not meet theset two or more design goals, modifying the initial design of theoptical metrology system and iterating the developing the model of theoptical metrology system, developing the prototype of the opticalmetrology system, collecting measured design goal data, performing a newcomparison of new measured two or more design goals to the set two ormore design goals, and modifying the initial design until there is amatch of the new measured design goals to the set two or more designgoals.
 2. The method for claim 1 wherein the workpiece is a wafer in asemiconductor application and wherein the optical metrology systemcomprises an optical metrology tool.
 3. The method for claim 2 wherein arange of capabilities of the optical metrology system comprisesmeasurement of one or more types of applications including onedimensional repeating structures, two dimensional repeating structure,and/or complex repeating structures comprising posts, contact holes,vias, islands, and concave or convex three dimensional structures, orcombinations of two or more thereof.
 4. The method of claim 2 whereinthe range of capabilities of the optical metrology system comprises useof two or more light sources, configurable angle of incidence ofillumination beams, configurable numerical aperture of the set ofoptical components, and two or more detectors.
 5. The method of claim 2wherein the design goals comprise two or more of accuracy of diffractionsignal measurement, repeatability of diffraction signal measurement,range of spot sizes of illumination beam that can be measured by theoptical metrology system, range of sizes of measurement spots,throughput in the number of workpieces measured per unit time, and typesof applications measured.
 6. The method of claim 2 wherein the two ormore design goals comprise tool-to-tool matching ranges for the opticalmetrology system to similar tools or tool-to-tool matching ranges forthe optical metrology system to a reference tool, reliability of theoptical metrology system expressed as up time or expressed as mean timebetween failures, time to develop libraries for extracting profileparameters, time to train machine learning systems for extractingprofile parameters, and cost of ownership of the optical metrologysystem.
 7. The method of claim 2 wherein the two or more design goalsinclude measurement of a spot size range of 32 by 32 micrometers orless, measurement accuracy of 98 percent or higher compared tomeasurements performed using an atomic force microscope or cross-sectionscanning electron microscope, and a throughput of 200 or more wafersmeasured per hour.
 8. The method of claim 2 wherein the two or moredesign goals include measurement accuracy of 98 percent or highercompared to measurements performed using an atomic force microscope orcross-section scanning electron microscope, a repeatability of themeasurement equal to or less than a 3-sigma of 4 nanometers, and a spotsize range of 32 by 32 micrometers or less.
 9. The method of claim 1wherein developing the optical metrology system model comprisesgenerating a model of the optical metrology tool using selectedcomponents with functional association with at least one of the set twoor more design goals.
 10. The method of claim 1 wherein the prototype ofthe optical metrology system includes two or more components coupled tosimulate optical, mechanical, and electrical couplings of the opticalmetrology system.
 11. The method of claim 1 wherein the prototype of theoptical metrology system includes a fully assembled optical metrologytool.
 12. The method of claim 1 wherein modifying the initial design ofthe optical metrology system and wherein the two or more design goalsinclude accuracy of diffraction signal measurements comprises: changingthe output intensity of two or more light sources; and/or changing bulbsused in the two or more light sources.
 13. The method of claim 1 whereinmodifying the initial design of the optical metrology system includesusing one or more detectors for each of two or more diffraction signals.14. The method of claim 1 wherein modifying the initial design of theoptical metrology system and wherein the two or more design goalsinclude repeatability of diffraction signal measurements comprises:selecting a first polarizer in an illumination path and a secondpolarizer in a detection path, wherein the first and second polarizersare configured to increase a signal to noise ratio of illumination anddetection beams respectively.
 15. The method of claim 1 whereinmodifying the initial design of the optical metrology system includesusing mirrors for focusing optics or using mirrors for focusing opticswith different quality coatings.
 16. The method of claim 1 whereinmodifying the initial design of the optical metrology system includes:using a selectable angle of incidence for one or more illumination beamsto optimize accuracy of diffraction signal measurements; or using higherefficiency grating and/or higher efficiency signal detector.
 17. Themethod of claim 1 wherein modifying the initial design of the opticalmetrology system includes: using configurable numerical apertures forfocusing optics and light sources; or using an optical path environmentpurged with nitrogen gas.
 18. The method of claim 1 wherein modifyingthe initial design of the optical metrology system includes adding alamp temperature control system configured to control the temperature ofthe illumination subsystem in a narrow temperature range.
 19. The methodof claim 1 further comprising: using the optical metrology system tomeasure a structure in the workpiece wherein a measured diffractionsignal is generated; and extracting at least one profile parameter ofthe structure from the measured diffraction signal using profileextraction methods that include a regression method, a library method,or a machine learning systems method.
 20. The method for claim 1 whereinthe workpiece is a wafer in a semiconductor application and wherein: theoptical metrology system is integrated in a fabrication cluster; or theoptical metrology system is part of a standalone metrology system.